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Water table mounding between subsoil drains

Serafini, Gregorio (2012) Water table mounding between subsoil drains. Internship Report, Murdoch University.



Perth is a rapidly growing city and the demand for urban land has been steadily increasing over recent years (D.P.I., 2011) and the meeting of environmental legislations for storm-water management and floodplain strategic management is the kernel around which a variety of skills and deep speculation revolves.

Technological advancement and multi-variable calculus have made possible the ability to predict how a given artefact will behave according to a defined series of variables in the environment within which the apparatus is expected to perform its duty. This is to introduce the key role that modelling resembles within the design process.

In geological terms, it is known with deposition that Perth is accommodated over a rather variegated spectrum of soil types that ranges from sand, silt, clayey sand and clay. Due to this multi-faceted nature there are no standard design guidelines for subsoil drainage systems at date. A calibrated mathematical model can readily facilitate the design process and decision making thanks to its ability to provide groundwater mounding height values for environments with different physical and hydrogeological characteristics.

Subsoil drainage is used throughout the world to control shallow groundwater levels to facilitate land use in both agricultural and urban development. The term subsoil implies that a buried pipe is used as opposed to an open drain, usually where land values are high.

The primary mechanism by which subsoil drainage functions is the provision of an outlet from a slotted pipe system such that groundwater can flow by gravity, according to Darcy’s law, towards the pipe thus lowering groundwater levels.

Subsoil drainage is usually constructed in parallel or sub-parallel lines, so that the water table mounds between the parallel drainage lines.

Critical parameters determining whether a subsoil drainage system operates as intended are: the soil permeability, the volume of water to be drained in unit time and drain spacing.

On the Swan Coastal Plain subsoil drainage has been used in urban developments where the water table has been shallow for decades, generally with success owing to the characteristically sandy soils and the relatively low rainfall in the South West of Western Australia.

This paper describes a subsoil drainage experimental site in the City of Armadale, instrumented to monitor water table mounding between a set of parallel subsoil drains in imported sand fill in 2009 and 2010. The paper describes the data collected together with application of a suitable model to represent the relevant components of water flux, and an application of that model to design the subsoil drainage systems on the Swan Coastal Plain.

It is the contention of this paper that a properly designed and calibrated model is to foresee the rise and decline of the perched water table under a given set of environmental conditions. Doing so, it will be possible to implement the Best Practice Technique (BPT) while addressing the need to avoid the oversizing of sand-fill requirements and consequent levitation of capital budget, maximise the security of assets and obviate the violation of buildings’ structural integrity.

In terms of literature review, it has been deliberately decided to report references that are relevant to the Western Australia scenario solely: this is justified by the unique morpho-geological arrangement of the Swan Coastal Plain; in fact, overseas regulators for urban development may enforce building codes, approval schemes and land sub-divisional requirements that are not parallel to the Australian decisional criteria for such practices.

Item Type: Internship Report (Bachelor of Engineering)
Murdoch Affiliation(s): School of Engineering and Energy
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